This invention relates to nuclear instrumentation for fast breeder reactors. More particularly, the invention relates to fast breeder reactor in-core nuclear instrumentation suitable for direct measurement of in-core neutron flux for such purposes as control of the degree of core fuel burn-up and detection of core anomaly.
When controlling the degree of burn-up of core fuel in a light-water reactor, the general practice is to measure in-core neutron flux distribution and operate control rods based on these measurements. The reason for this is as follows: If an anomaly should develop inside the core, neutron flux is affected first, after which the fuel produces heat causing a rise in the temperature of the coolant. This appears as a change in temperature at the outlet of the core. Therefore, in order to grasp these conditions, a quicker response is attained by measuring neutron flux, which is a primary quantity, rather than temperature change, which is a secondary quantity.
In the case of a fast breeder reactor, the neutron flux distribution is simpler than that seen in a light-water reactor. It has been contemplated, therefore, to measure the change in in-core neutron flux outside the core. As shown in FIG. 6, neutron flux measuring units 2 are disposed on the inner side of a reactor vessel chamber wall 23 and on the outer side of a core 24 to measure the change in neutron flux.
In this conventional nuclear instrumentation for a fast breeder reactor in which the neutron flux measuring units 2 are arranged on the outer side of the core 24, accurate measurement can be performed with regard to small and medium size reactors such as experimental and prototype reactors. However, accurate measurement is not always possible in demonstration or commercial reactors having a large sized core. Moreover, when a heterogeneous core (heterogeneous in the axial and diametric directions) is employed in a demonstration or commercial class reactor, measuring in-core neutron distribution accurately is difficult with the conventional nuclear instrumentation.
Furthermore, in order to measure in-core neutron flux in a commercial-class light-water reactor, a special instrumentation channel generally is provided in the core. However, in order to apply this to a fast breeder reactor, a major modification in core design is required.